The present invention is directed to polymeric materials and, more particularly, to thermoplastic polyester compositions having high dimensional stability at elevated temperatures with improved toughness, which especially are useful in food-grade applications such as dual-ovenable containers.
Polyesters are polymeric materials typically made by a condensation reaction of dibasic acids and dihydric alcohols. Common examples of polyesters include alkylene terephthalate and naphthalate polymers such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polycycloterephthate (PCT), polycycloterephthatlic acid (PCTA), (poly)ethylene-co-1,4-cyclohexanedimethylene terephthalate (PETG), and polytrimethylene terephthalate (PTT). Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are examples of polyesters having excellent barrier properties, excellent chemical resistance, good dimensional stability at room temperature, and high abrasion resistance. However, polyalkylene terephthalates tend to become brittle upon crystallization, especially upon thermal crystallization, and are not dimensionally stable at temperatures above their glass transition temperature (Tg). As a consequence, non-oriented, thermally crystallized polyalkylene terephthalates have poor ductility, poor impact resistance, and poor heat resistance, which limit the utility of the polymer in many applications.
Several attempts have been made to improve the impact properties of polyalkylene terephthalates, including the addition of various impact modifiers as described in U.S. Pat. Nos. 4,172,859, 4,284,540, and 4,753,980. U.S. Pat. No. 4,753,980 discloses the use of an ethylene/n-butylene acrylate/glycidyl methacrylate ter-polymer to produce toughened polyester.
Another class of polymer having widespread utility is polyethylenes, which are ethylene-based polyolefin polymers. Polyethylenes most often are linear but also can be branched. Linear polyethylenes typically are classified by density, e.g., low-density polyethylene (LDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), and the like. Polyethylenes exhibit good toughness, low moisture absorption, high chemical resistance, excellent electrical insulating properties, low coefficient of friction, and ease of processing. However, polyethylenes have poor load-bearing and gas barrier characteristics, and relatively poor heat resistance properties.
Numerous attempts have been made to combine polyalkylene terephthalates and polyethylene polymers in efforts to realize the above-described useful properties of each of the two types of polymer. However, polyalkylene terephthalates and polyethylenes are highly incompatible because of their significantly different physical and chemical properties, such as solubility, surface tension, and polarity. Combining the two polymers typically results in a phase-separated blend exhibiting poor mechanical properties, especially impact properties. In addition, each of the polymers retains its own thermal properties. Therefore, the phase-separated blend thermally degrades at the lower degradation temperature of the two polymers. Such a blend is entirely unsatisfactory.
Various thermoplastic elastomers have been proposed to improve the compatibility of polyalkylene terephthalates and polyethylenes. Traugott et at., J. Appl. Poly. Sci., 28, 2947 (1983), describes blends of polyethylene terephthalate and high density polyethylene (PET/HDPE) which are said to exhibit high ductility. The blend compositions utilize up to 20 weight percent of a styrene/ethylene-butadiene/styrene tri-block co-polymer (SEBS) or an ethylene-propylene co-polymer as a compatibilizing thermoplastic elastomer.
The relatively large concentrations of the thermoplastic elastomers which are required to improve compatibility, however, can diminish other desirable properties of the polymer blends, most notably impact properties and thermal stabilities. U.S. Pat. No. 5,436,296 describes a co-polymer of a C2-C10 alpha-olefin and a glycidyl or isocyanate group-containing functional compound which is said to compatibilize thermoplastic blends of polyalkylene terephthalates and polyethylenes while improving impact properties and heat resistance. The thermoplastic blend is described as a continuous matrix of polyalkylene terephthalate with polyethylene domains dispersed therein.
Food grade containers which can be used for cooking or reconstituting foodstuffs are of particular interest. Such containers, whether disposable or intended for re-use, typically are heated to temperatures exceeding 250xc2x0 F. and must be capable of being heated to at least about 350-400xc2x0 F. without significant distortion of the rigid package if the container is to be considered ovenable. Food containers made of polymeric materials are used in a wide variety of applications. For example, foamed polystyrene is widely used in making hot drink cups. It is also used in making xe2x80x9cclam shellsxe2x80x9d which are used by the fast food industry as packages for hamburgers and other types of sandwiches. One drawback associated with the use of polystyrene is the possible migration of residual styrene into food products, especially when the container is reheated, e.g., by a microwave oven. There are strict limitations on the quantities of styrene and various other plastics components which may be liberated from a plastic container into food in the container.
The wide spread popularity of microwave ovens for home use has initiated interest in food trays which can be used in either microwave ovens or convection ovens. Such trays are of particular value as containers for frozen prepared foods. It is important for such trays to have good impact strength and dimensional stability at both freezer and oven temperatures. Of course, it also is important for such trays to be capable of withstanding rapid heating from freezer temperatures of about xe2x88x9222xc2x0 F. to oven temperatures exceeding about 250xc2x0 F.
Containers which are capable of being heated in either convection ovens or microwave ovens are sometimes described as being dual-ovenable. Polyesters are highly suitable for use in making such dual-ovenable containers. However, it is important for the polyester to be in the crystalline state rather than the amorphous state in order to achieve satisfactory high temperature stability. Normally, polyesters will undergo crystallization by heat treatment at elevated temperatures and the crystallites formed will remain substantially stable up to near the melting point of the polyester. As a general rule, dual-ovenable containers which are comprised of polyester will be heat-treated to attain a crystallinity of higher than about 20%.
Injection molding and thermoforming are widely known methods for forming thermoplastic polyester articles. In injection molding, the polyester is heated above its melting point and injected under sufficient pressure to force the molten polyester to fill the mold cavity. The molten polyester is cooled in the mold until it is rigid enough to be removed. Injection molding of a polyester composition containing 0.5% to 10% by weight isotactic polybutene-1 is described in U.S. Pat. No. 3,839,499. This injection molding method, however, generally is not satisfactory for the production of thin walled articles, such as dual-ovenable trays, due to flow lines and layering which develop during the filling of the mold which lead to non-uniform properties, surface irregularities, and warping of the finished article.
Thermoforming is another process which is used commercially in the production of polyester articles. It is a particularly valuable technique for use in producing thin walled articles, such as dual-ovenable food trays, on a commercial basis. In thermoforming, a sheet of preformed polyester is preheated to a temperature sufficient to allow deformation of the sheet. The sheet is then made to conform to the contours of a mold by such means as vacuum assist, air pressure assist, or matched mold assist. The thermoformed article produced is normally heat treated in the mold in order to attain a crystallinity of at least about 20%.
Crystallization rates generally can be improved by including a small amount of a nucleating agent in polyester compositions. For example, U.S. Pat. No. 3,960,807 describes a process for thermoforming articles from a polyester composition having an intrinsic viscosity (I.V.) of at least 0.75 which is comprised of (1) a crystallizable polyester, (2) a crack stopping agent, preferably a polyolefin, and (3) a nucleating agent. Polyester articles which are made utilizing such compositions generally have improved mold release characteristics and improved impact strength. Additionally, the utilization of such modified polyester compositions results in faster thermoforming cycle times due to the associated faster rate of crystallization. U.S. Pat. No. 4,981,631 describes thermoforming a substantially amorphous cellular sheet which is comprised of (a) from about 94 to 99 wt % polyethylene terephthalate having an I.V. of at least 0.7 dl/g, (b) from about 1 to 6 wt % of at least one polyolefin, and (c) a sufficient amount of inert gas cells to provide the cellular sheet with a density within the range of about 0.4 to 1.25. Thermoforming is said to be carried out in a heated mold for a time sufficient to achieve a crystallinity of from about 5% to 45%.
U.S. Pat. No. 5,409,967 describes an amorphous, aromatic polyester such as PET having an initial I.V. of at least 0.7 dl/g blended with an impact modifier. The impact modifier is described as a core-shell polymer with cores comprised mainly of rubbery polymers of diolefins and vinyl aromatic monomers, and shells comprised mainly of styrene co-polymers such as styrene and hydroxyalkyl (meth)acrylate. The impact modifier is said to substantially increase impact strength of amorphous, aromatic polyesters without detracting from clarity (transparency).
Polyesters presently used in dual-ovenable trays generally are required to have an I.V. of at least 0.95 dl/g in order for the heat-set articles to be food-grade, to have sufficient impact strength at low temperatures (e.g., as in a freezer), and to have sufficient toughness and dimensional stability over a broad temperature range.
It would be desirable to develop a polyester composition having improved molding properties, especially one which is a thermoplastic composition having high dimensional stability, and high temperature resistance, which is thermally stable (e.g., capable of being reprocessed or recycled without loss of toughness or generation of degradation by-products), and which is useful in food-grade applications such as in making dual-ovenable containers. It would be especially desirable to develop a toughener additive which permits the use of polyesters having lower I.V. (e.g., less than 0.95) in heat-set articles which are food-grade, which have high dimensional stability and high temperature resistance, which are thermally stable, and which additionally retain toughness, especially at low temperatures.
According to one embodiment, the present invention is directed to a polyester thermoplastic composition comprising a bulk polymer, an additive in a concentration from about 4 wt % to about 40 wt %, and a compatibilizer/emulsifier/surfactant (CES) in a concentration from about 0.1 wt % to about 8 wt %, based on the total weight of the composition. The bulk polymer comprises an alkylene terephthalate or naphthalate polyester such as polyethylene terephthalate (PET). The additive comprises an amorphous or substantially amorphous co-polymer of ethylene and at least one co-monomer that forms polar portions, such as an acrylate, e.g., methacrylate, butylacrylate, ethyl acrylate, ethylhexyl methacrylate, or a mixture thereof. The CES comprises a grafted or backbone co-polymer or ter-polymer of ethylene and a glycidyl acrylate or maleic anhydride, and optionally methacrylate, butylacrylate, ethyl acrylate, ethylhexyl methacrylate, or mixtures thereof.
According to another embodiment of the present invention, an additive for providing toughness to a thermoplastic composition consists essentially of a substantially amorphous ethylene co-polymer with an acrylate co-monomer concentration of from about 7 wt % to about 40 wt %, based on the total weight of the co-polymer, and optionally a core-shell toughener.
According to another embodiment of the present invention, a container comprises a molded thermoplastic composition which can be heat set and which includes a bulk polymer, an additive, and a compatibilizer/emulsifier/surfactant (CES). The container can contain one or more solid layers, cellular layers, or a combination thereof.
According to yet another embodiment of the invention, a food-grade cooking container can be made by thermal crystallization of a non-oriented or substantially non-oriented composition comprising a bulk polymer of an alkylene terephthalate or naphthalate having an intrinsic viscosity of less than 0.95, preferably less than about 0.90, more preferably less than about 0.85, and even more preferably less than about 0.80, wherein the bulk polymer is present in a concentration of at least about 30 wt %, preferably at least about 45 wt %, more preferably at least about 55 wt %, and even more preferably at least about 60 wt %, based on a total weight of the composition. The food-grade container can contain one or more solid layers, cellular layers, or a combination thereof.
According to a further embodiment of the invention, a method of imparting elasticity to an alkylene terephthalate or naphthalate polyester comprises adding to the bulk polymer an effective amount of a substantially amorphous ethylene co-polymer with an acrylate co-monomer concentration of from about 7 wt % to about 40 wt %, based on a total weight of the ethylene co-polymer. Suitable effective amounts of the ethylene co-polymer typically range from about 4 wt % to about 40 wt %, based on the total weight of the bulk polymer and ethylene co-polymer.
According to a further embodiment of the invention, a method of improving processibility of an alkylene terephthalate or naphthalate polyester comprises adding to the bulk polymer an effective amount of a substantially amorphous ethylene co-polymer with an acrylate co-monomer concentration of from about 7 wt % to about 40 wt %, based on a total weight of the ethylene co-polymer. Suitable effective amounts of the ethylene co-polymer typically range from about 4 wt % to about 40 wt %, based on the total weight of the bulk polymer and ethylene co-polymer.
According to another embodiment of the invention, a food-grade thermoplastic composition comprises a bulk polymer, an additive, and a compatibilizer/emulsifier/surfactant (CES).
According to yet another embodiment of the invention, a food-grade cooking container comprises a bulk polymer, an additive, and a compatibilizer/emulsifier/surfactant (CES). The food-grade container can contain one or more solid layers, cellular layers, or a combination thereof.
According to a further embodiment of the invention, a compatibilizer/emulsifier/surfactant (CES) for use with polyester compositions is selected from ethylene/maleic anhydride co-polymer, ethylene/glycidyl methacrylate/ethylhexyl acrylate ter-polymer, ethylene/maleic anhydride/methacrylate ter-polymer, ethylene/maleic anhydride/ethylacrylate ter-polymer, ethylene/maleic anhydride/butylacrylate ter-polymer, ethylene/maleic anhydride/ethylhexylacrylate ter-polymer, and mixtures thereof.
Preferred thermoplastic compositions of the present invention exhibit properties which are especially desirable in food-grade applications. One such property is a relatively low to medium I.V. of PET, which improves extrusion and molding of a heat-set product with improved toughness. Other such properties include high dimensional stability, high temperature resistance, toughness, processibility, and improved molding detail. Further, the composition is thermally stable, resulting in low extractives and less degradation and ensuring food-grade compliance. Such properties render the thermoplastic composition of the present invention especially suitable for use in dual-ovenable containers, which require high dimensional stability and sealability for high-speed packaging and distribution of frozen or refrigerated food products. The ability to use lower-I.V. polyesters permits more economical production of heat-set products having high dimensional stability and resistance to degradation over broad temperature ranges.